I've been using the Navy Body Fat Calculation method for about two months in conjunction with an exercise regimen. It's done it's job as far as telling me that I'm loosing body fat and gaining lean muscle in it's place, but I still wonder how accurate it is and can be.

From everything I've read on the net, the general consensus is that it's accurate to within 3% of what Hydrostatic Weighing gives you, but I've noticed something about it's output, namely that my neck measurement has more of a (reverse) hold on what my BFP is than my waist measurement does.

For example:

If my waist measurement increases size 1/8th of an inch (which would indicate a gain of BFP), but my neck measurement increases by 3/8ths of an inch, then my BFP is calculated as losing .61 percentage points.

On the other hand, if my waist measurement stays the same (which indicates no change), and my neck measurement decreases by 1/2 inch, my BFP is calculated as gaining a half a percentage point.

I understand I'm being nit-picky here, as it is telling me that over the past two months I've lost about 4% (coming up on 5%) in BFP, but wouldn't my waist have more weight in the calculation of what my body fat is doing than my neck? The way it is now it seems kind ass backwards, pardon the french.

I couldn't find a site that actually explains these equations, but I can make some educated guess about the rationale behind them. The first term probably gives you a an estimate of body fat based on a couple circumference measurements. However, this estimate lacks precision and accuracy, so it needs to be corrected. The second term probably corrects for height (so that tall people don't get punished for have proportionate girths), and the last term is probably an experimentally-justified constant that corrects the result of the equation to it's as close as possible to actual body fat levels.

If you check out the first term, you can see that waist and waist/hip measurements are used to estimate body fat of men and women respectively. This makes sense since these areas correspond to primary areas of fat deposition for each sex. You'll also notice that neck circumference is actually subtracted, so it makes sense that gaining more girth in your neck than your waist would cause a decrease in estimated body fat. I think neck circumference is being used to correct for your lean body mass so you don't get punished for having big muscles and bones. Maybe this is because necks don't usually have that much fat. The underlying assumption seems to be: if you have a larger neck girth, you have more muscle, bigger bones, etc., which shouldn't be counted as fat. If your neck circumference increases more than your waist circumference, the assumption is that the increase in your waist was due to muscle, not fat.

If the method is accurate within 3%, it must work pretty well for most people, but there are obvious ways the results could be skewed. For example, if a male only did exercises that beefed up his neck without adding much muscle to his torso, the equation would probably underestimate his body fat. Or, if a female had muscular thighs, but a weaker upper body and neck (typical of runners and dancers), the equation might overestimate her body fat. People with uncharacteristic fat distribution would also have skewed results.

For men, the correlation coefficient was 0.885 and the standard error
of measurement is 3.15% fat. The mean difference between measured and
predicted values is -0.833% fat, the predicted values being greater,
on average, than the measured four-compartment percent fat.
Comparisons with two-compartment fat values reveal a correlation
coefficient of 0.89, a standard error of measurement of 3.37% fat, and
a mean difference of –1.25% fat; again, with the predicted value being
greater than the measured. For women, comparisons with four-
compartment fat provide a correlation coefficient of 0.89, a standard
error of measurement of 3.12% fat, and a mean difference of –2.00%
fat. Comparisons with two-compartment fat produced a correlation
coefficient of 0.82, standard error of measurement of 4.15% fat, and a
mean difference of –3.22% fat. It is noteworthy that, in this newer
sample, the Navy equations are better predictors of the percent fat
values from four-compartment analysis than they are of values from a
two-compartment analysis.

I think the basic assumption in the Navy calculation is that fat doesn't accumulate much, if at all, on the neck. So if your neck is getting bigger, then you're putting on muscle, and therefore the same waist measurement must come from more muscle mass and less fat. (Whether muscle really builds on the neck, or the neck measurement is a proxy for the overall scale of your body, is beyond me.)

According to About.com, you are supposed to measure the neck at the Adams Apple on a downward angle which is where the traps start.
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Salsero69Apr 21 '11 at 4:22

1

@Salsero69: I'm not sure what that has to do with my answer. I found the same information from my source, so I am not doubting your statement, I am just not sure what you are pointing at.
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jpreteApr 21 '11 at 6:04

1

I think I was too tired last night when I misread your answer. Sorry. :-(
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Salsero69Apr 21 '11 at 16:56